1. Field of the Invention
The present invention relates to a method and apparatus for tuning narrow band wave guide reflectors arranged on a substrate and wherein wave guide layers are provided having periodic perturbations or their direct optical equivalent.
2. Description of the Prior Art
Reflectors for wave guide optical layers cannot be achieved by use of conventional construction techniques with dielectric layer systems. Only in special cases when geometrically thick wave guides are used is it possible to insert dielectric end mirrors or reflectors into the light path of the wave guide at a later time with the aid of a hybrid technique. Such technology is very expensive, however.
Alternatively, there is the possibility of using distributed feedback coupling (DFB) and distributed Bragg reflection (DBR) by way of periodic perturbations of the wave guide in the light expansion direction. The periodic perturbation is a localized variation of the effective index of refraction EQU n.sub.eff = .beta./k = c/.nu.
for the mode considered, i.e. for the relation of light speed in a vacuum to that of light speed of this mode in the wave guide. The effective index of refraction n.sub.eff is linked at the reflector with a grating constant g related to the periodic perturbation by way of the Bragg condition EQU g = .lambda./2 n.sub.eff.
Such small grating constants, however, can at the present time only be produced holographically in the wave guide for the optical wavelength range. The grating constant may be produced directly as a period thickness variation of a photo lacquer superstrate or with the aid of successive photolithographic method techniques such as etching processes, evaporation technique, diffusion and implantation.
With the above techniques, however, it is difficult to precisely maintain the necessary tolerances for the grating constant g.